1. Field of the Invention
This invention relates generally to multi-junction interferometer devices which are to be used in high-speed logic circuits. It further relates to symmetrically fed, multi-function interferometers wherein resonances occur due to the interaction between the Josephson junction capacitances and an inductance which interconnects them. Still more specifically, it relates to interferometer structures in which the number of resonant peaks present is determined by the number of meshes in the interferometer device. In a preferred embodiment, a two-junction interferometer has a single mesh formed by the pair of junctions interconnected by a main inductance and a small value of stray inductance. To achieve suppression of current resonance peaks in the device which is intended to operate in a below the gap regime, a low inductance resistance is disposed so that it effectively shunts the main inductance of the device.
In other embodiments, where multiple meshes are formed by pairs of junctions and their connecting main inductances, each of the latter is similarly shunted by a low inductance resistance which effectively suppresses the current resonance peaks to insure that the interferometer switches to the desired below-the-gap voltage.
2. Description of the Prior Art
U.S. Pat. No. 3,906,538, Ser. No. 422,960, filed Dec. 7, 1973 entitled "Techniques for Minimizing Resonance Amplitudes of Josephson Junctions" and assigned to the same assignee as the present invention, deals with resonant conditions in a single rectangular Josephson junction. The preferred embodiment reduces the resonant conditions by shaping the junction so it is no longer rectangular. The patent does not specifically deal with resonances in multijunction interferometers. The teaching cannot be extended to the suppression of macroscopic resonances in multijunction interferometers.
U.S. Pat. No. 3,879,715, Ser. No. 429,412, filed Dec. 28, 1973 entitled "Damped Josephson Junction Memory Cell with Inductively Coupled Resistive Loop" deals with the damping of memory cells for effective operation by inductively coupling a resistive loop to the memory cell. The inductively coupled resistor effectively reduces the junction resistance, R.sub.j, from a relatively high value to a lower value. Apart from the fact that this patent does not deal with multiple junction interferometers, the expedient of inductively coupling a resistor to an interferometer would not suppress resonances in multi-junction devices because an effective series inductance would be provided which would not allow effective suppression of resonances.
U.S. Pat. Nos. 3,705,393, Ser. No. 51,057, filed June 30, 1970 entitled "Superconducting Memory Array Using Weak Links" deals with memory cells formed of a superconducting ring and a weak link interposed therein. The patent indicates at Col. 3, lines 21-33: "If the capacitance of the Josephson device is too high, there will be oscillation of the energy stored in the capacitance between the electric field associated with the capacitance and the magnetic field associated with the inductance of the superconducting loop."
"In order to reduce these oscillations, the capacitance of the loop must be small and there must be damping associated with the Josephson device. The damping is related to the geometry of the Josephson device and, if required, a bridge between the superconducting elements of the device can be provided to affect damping."
At col. 7, lines 14-20 , the oscillations are controlled as follows: "The damping is a function of the Josephson device and can be somewhat controlled by adding resistive material to the Josephson barrier or by bridging the Josephson Junction. For instance, it is possible to create a metal bridge across the Josephson barrier to influence the resistance of the junction."
The above approach effectively increases damping by decreasing R.sub.j. While the oscillations referred to are not of the same character as macroscopic resonances, even if one were to apply the teachings of this patent to interferometer logic circuits to provide low junction resistances, R.sub.j, therein, the resulting device would not be effective as a logic circuit because insufficient current would be diverted to an associated load to permit operation as a logic device.
IBM Technical Disclosure Bulletin, Vol. 16, No. 6, Nov. 1973, p. 2020, in an article entitled "Josephson-Junction with Negligible Hysteresis" by W. W. Jutzi, the author indicates that resonances generated with a control field can seriously limit the voltage swing across the junction of logic circuits. He indicates that resonances can be avoided by loading the junction with ohmic resistance. This is accomplished by forming confined zones of normal conductivity in the superconducting electrodes of a Josephson junction. This article does not deal with multi-junction devices, and to the extent the teaching thereof could be extended to multi-junction devices, one would be led to resistively load each junction electrode in the same manner as shown, thereby reducing the output current and rendering such devices ineffective as logic devices.
U.S. Pat. No. 3,676,718, Ser. No. 128,445, filed Mar. 26, 1971, entitled "Supercurrent Structures Utilizing Mobil Flux Vortices" in FIG. 5 thereof shows a structure similar to the interferometers utilized in the present application. While two junctions are shown, the structure is not an interferometer and loop 40 is utilized to form a "hole" which creates a preferred vortex location. In addition, the device is not properly symmetrically current fed nor does it incorporate or even concern itself with the use of a resistance across the main inductance thereof to suppress resonances.